Abstract:

Carbon-in-silica composite films have been fabricated using sol-gel synthesis of
silica and carbon precursor materials, followed by carbonization in an inert
atmosphere. Four categories of samples were fabricated and investigated. These
were the tetraethyl orthosilicate-only (TEOS-only), methyl-triethoxysilane (MTES),
acetic acid anhydride (Ac2O) and soot (SOOT) samples. The carbon-in-silica films,
spin-coated on aluminium and steel substrates, have been investigated for selective
solar absorber functionality. Optical measurements were performed on these
samples to determine solar absorptance and thermal emittance of each. The
morphology of the films was studied by scanning electron microscopy (SEM). The
fine structure of the samples was studied by cross-section high-resolution
transmission electron microscopy (X-HRTEM). Electron energy-loss spectroscopy
(EELS) mapping was used to determine the carbon distribution. The highest solar
absorptance attained with these coatings is 0.95 and the least emittance obtained is
0.1. An optimum performance of 0.88 for solar absorptance and 0.41 for thermal
emittance has been achieved in a single sample. The sol-gel technique produced
films with very flat surfaces and uniform thicknesses of the order of 1 μm. The fine
structure showed homogeneous mixing of the carbon and silica in the TEOS-only
samples while the separate additions of MTES and Ac2O resulted in segregation of
composition of the silica and carbon at nano-scale level. The addition of 20 wt.%
MTES or 15 wt.% Ac2O to the TEOS-only sols also helped to reduce the cracks in
the TEOS-only samples, although Ac2O seemed to be a better choice of the two. The
addition of soot in place of the carbon precursor did not yield a net advantage. This
technique of production of selective solar surfaces is promising to compete
favourably with other techniques, especially for its low cost of running and of initial
investment.